1. Field of the Invention
The present invention relates to an optical switch and a using method therefor, and more particularly to an optical switch suitable for a node in a photonic network using wavelength division multiplexing (WDM).
2. Description of the Related Art
The development and commercialization of a wavelength division multiplexing (WDM) system are proceeding as a communication system that can greatly increase a transmission capacity. To construct a large-scale photonic network by connecting WDM systems, there has been examined a ring type or mesh type network obtained by connecting nodes through optical fibers in the form of a loop or mesh.
In the ring type network, a transmission capacity in the loop increases with an increase in scale of the network. However, in each node, it is sufficient to perform processing using a relatively small-scale optical switch. To the contrary, in the mesh type network, a transmission capacity in each route is small, but it is necessary to perform processing using a large-scale optical switch in each node.
In a point-to-point link system, an electrical switch is conventionally used to extract lower-order signals in the node. By substituting an optical switch for the electrical switch, a cost in the node can be reduced.
Thus, the development of a large-scale optical switch is a key technology in constructing various types of networks.
A waveguide type optical switch is known as a conventional commercialized small-scale optical switch. The waveguide type optical switch includes a switch element and fiber arrays for inputs and outputs connected to the switch element.
For example, an optical switch referred to as a PILOSS type optical switch (Japanese Patent Laid-open No. 63-500140) has been developed to eliminate variations in loss according to the number of switch cells through which light is transmitted. This optical switch is configured by arranging N2 switch cells each having two inputs and two outputs at the lattice positions of a matrix with N rows and N columns and suitably connecting the inputs and the outputs of the switch cells so as not to cause the path dependence of loss.
As an optical switch which can enlarge an integration scale with a low loss, there has recently been developed a bubble type optical switch configured by forming a bulk at each crossover of crossing type optical waveguides and generating a bubble in the bulk to thereby obtain a total reflection condition. In each switch cell, the transmission and total reflection of light are switched to thereby obtain a switch function with two inputs and two outputs.
On the other hand, a configuration of spatially switching light is considered as a traditional technique. By using a reflection mirror as an element for changing an optical path, the problems in performance of the waveguide type optical switch, such as on/off ratio and crosstalk can be almost eliminated. However, such a space switch is large in volume, and it is therefore difficult to increase the scale of the switch from the viewpoint of size.
To break through such circumstances, there has recently been developed a technique of reducing the size of this space switch by using a semiconductor technology. This technique is referred to as MEMS (Micro Electro Mechanical System), and it is also called optical MEMS in the case of application to the field of optics.
The optical switch using MEMS has a plurality of small mirrors formed on a substrate by a semiconductor fabrication technique, and performs switching of optical paths by selectively raising these mirrors by static electricity.
Information on MEMS may be provided by IEEE Photonic Technology Letters, Vol. 10, No. 4, April 1998, pp. 525-527.
To increase the scale of a waveguide type optical switch, the yield of each switch cell itself formed on the switch element must be increased. However, increasing the yield is relatively difficult because of narrow manufacturing tolerances. Accordingly, in increasing the scale of the waveguide type optical switch, it is necessary not only to improve the yield by improving the manufacturing method, but also to remarkably improve the performance of the switch element.
In the bubble type optical switch, switching is performed by using the principle of total reflection in each switch cell. Accordingly, the angle of crossing of the two optical waveguides connecting the two inputs and the two outputs in each switch cell is as large as about 90°, causing an increase in switch size. In other words, if the bend radius of curvature of an optical waveguide connecting the adjacent switch cells arranged on the outermost side is reduced, the loss in this optical waveguide is increased. Therefore, the bend radius of curvature of this optical waveguide must be set to a sufficient amount. In connection with this setting, the pitch of the switch cells is determined, resulting in an increase in switch size.
In the waveguide type or bubble type optical switch, there is a case that crossover portions of the waveguides are required on the input and output sides, causing an unignorable loss.
Further, in the MEMS type optical switch, there is a possibility that the number of reflections on the mirrors may be different according to path in some mode of operation. Accordingly, in the case that the reflection loss by the mirrors is unignorable, there arises a problem that a path-dependent loss is produced according to a difference in number of reflections on the mirrors.